Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/636156
28 | February 2016 gauge face. The plunger should feature a small roller bearing at the tip to provide a consistent reading. The plunger tip should be placed about 1/2-inch inboard from the outer edge of pad contact. Slowly rotate the rotor and locate the low spot, then zero the gauge again. Using a Sharpie, make a reference mark on the disc at the lowest reading location. Slowly rotate the rotor, observing the gauge, noting the highest reading. The dif- ference represents the amount of runout. Vehicle manufacturer specifcations may vary, but as a rule of thumb, the maximum allowable runout is about 0.002-inch to 0.001-inch for most applications. Depending on the rotor design, you may be able to correct for runout using an on-car lathe, or you may need to simply replace the rotor. However, before replacing a rotor that you suspect of having excess runout, make a matchmark on the rotor hat and a corresponding wheel stud, then remove the rotor and reinstall at the next clockwise position and re-check runout. You may have a situation where toler- ances between the rotor and the hub are creating the excess runout. Continue to relocate the rotor on the hub, checking runout with each change. You may be able to install the rotor with the high point of the rotor aligned with the low point of hub runout. Measure disc thickness, but avoid making any decision based on only one measure- ment location. Measure for disc thickness in The Chassis Whenever checking rotors for lateral runout, always install all wheel fasteners and torque to specifcation. Installing only a few fasteners can easily result in a mis-reading that can indi- cate a runout issue where none exists. You must duplicate the installed wheel condition. Photo courtesy of Raybestos Brakes